Conventional automatic transmissions used in vehicles include a torque converter, a multi-stage gear shift mechanism connected to the torque converter, and a plurality of friction elements actuated by hydraulic pressure for selecting one of the gear stages of the gear shift mechanism.
A hydraulic control system for controlling the automatic transmission operates by selectively supplying hydraulic pressure, the flow of which is generated by a hydraulic pump, to engage or disengage each friction element by a plurality of control valves such that shifting can be performed automatically and appropriately for the driving state of the vehicle.
Such hydraulic control systems generally comprise a pressure regulating controller for controlling the hydraulic pressure generated by the hydraulic pump, manual and automatic shift controllers for selecting a shift mode, a hydraulic pressure controller for controlling shift quality and shift response, a damper clutch controller for actuating a damper clutch of the torque converter, and a hydraulic pressure distributor for supplying an appropriate amount of hydraulic pressure to each of the friction element.
The torque converter includes an impeller connected to an output shaft of an engine so as to be rotated thereby, a turbine splined to a transmission input shaft, and a stator disposed between the impeller and the turbine which redirects fluid flow back to the same direction as impeller rotation to assist impeller rotation.
Automatic shifting is realized by changing shift ratios in a planetary gearset by selectively operating friction elements corresponding to each shift range. The friction elements are selectively operated by the changing of the flow of hydraulic pressure thereto using the plurality of valves of the hydraulic control system. The valves receive hydraulic pressure through a manual valve which is indexed with a selector lever operated by the driver to realize port conversion. Further, through the ON/OFF and duty control of solenoid valves by a transmission control unit (TCU), hydraulic pressure distribution by the hydraulic pressure distributor is changed and friction elements are selectively operated thereby, realizing the control of shift stages.
However, in such a conventional hydraulic control system, shift shock is most severe when shifting into a drive D range after reversing, where a drive D2 creep state initially occurs before the accelerator is depressed by the driver. The shift shock is caused by the friction elements interlocking the friction elements in the drive D2 creep state.